What was the book's subject? I've always managed to convince myself that the poor performance (relatively to values such as I see you have quoted) is because my market is somewhat niche, but it may not be the case, so any info on such self-published success would be useful for me.
It's the definition of a money topic. But I know that there are people who make a fortune with small cook books.
I know some other very profitable niches, but I can't talk about them in detail - competition is already extreme. But trust me: There's a ton of money in most topics.
OT: About daytrading (most people think it's basically a scam) - I know people who do daytrading on a daily basis (what a pun) and who live relatively comfortable investing $25-40k. I know a guy who went from 12k to 80k with one investment (was a 2-year investment, though). He's not gambling, he is a trading nerd reading the news and all company reports constantly. But yes, 99% of daytraders lose money because they start to become greedy or have no discipline.
...well I guess the important question is whether or not you think you have a high chance of showing up at a high rank for search terms with enough traffic, whether you have a chance of being featured prominently by amazon's recommender engine etc etc. If you want to make that work for you, you probably have to play a game like being a demand-driven content farm gaming Amazon in a way akin to how SEO tries to game Google.
Maybe there's money in it, but it's probably not the kind of content I would either want to produce or consume.
For most authors, the point of departure is probably, more idealistically, some well written content, maybe with an audience that's a bit niche, and the desire to find a way to get remunerated fairly on the effort that went into producing the content. It sounds to me like that's not really what Amazon is offering.
His book is fairly good and well-written, but I know what you mean.
I'm currently helping an author who wants to write a book and make some money with it (she wrote books in the past and worked with publishers). I explained to her how the self-publishing business works and she also had the same reaction. It's possible to write high-quality content and get fairly compensated, but you should definitely know that most people seem to be content with sub-par books. The average quality is really bad and people seem to like it (e.g. another very profitable niche: erotic books, Shades of Grey is just one of many of them).
Amazon is very generic. If you want to build an audience, you should stick to Facebook, Instagram, Pinterest, Snapchat, Slack/chat groups, write a blog, do vlogs and videos, organize local and offline groups, events and other methods. You can then build a whole community around a niche and sell way more than a book. This is also what I've recommended to her because this is how you build high-quality communities.
